A vast number of people are affected by conditions that result in profound muscle weakness or impaired motor control. For example, people with severe muscle weakness from neurological injury, such as hemiparesis from stroke, often have substantial movement limitations, and for many people, sit-to-stand motion becomes increasingly difficult with age.
One of the aims of rehabilitation after stroke is to improve the walking function. However, equipment available to facilitate this is severely limited. Several lower extremity rehabilitation machines have been developed recently to help retrain gait during walking. Lokomat® is an actively powered exoskeleton, designed for persons with spinal cord injury. The persons use this machine while walking on a treadmill. Mechanized Gait Trainer® (MGT) is a single degree of freedom powered machine that drives the leg to move in a prescribed gait pattern. The machine consists of a foot plate connected to a crank and rocker system. The device simulates the phases of gait, supports the subjects according to their abilities, and controls the center of mass in the vertical and horizontal directions. Auto-Ambulator® is a rehabilitation machine for assisting individuals, with stroke and spinal cord injuries, in leg motion impairments. This machine is designed to replicate the pattern of normal gait.
Sit-to-stand (STS) is one of the most common daily activities. It is a pre-requisite for other functional movements that require ambulation and is mechanically demanding. In the United States, an estimated two million people over age 64 have difficulty in rising from a chair. Functional electrical stimulation (FES) of muscles has been used to assist disabled individuals with STS motion, in particular to assist a paraplegic person to stand from a wheelchair. Handle reactions have also been used as a measure of stimulation of leg muscles during standing up. Powered robotic assistive devices that may incorporate FES have also been designed for standing-up training. KineAssist is a robotic device for gait and balance training. It is a microprocessor controlled, motor actuated device that provides partial body weight support and postural torques on the torso.
The use of these gait-training and sit-to-stand machines is limited in that they require external power to function, posing increased risk to the user, and require supervisory staff for safe use. Additionally, they only move persons through predetermined movement patterns rather than allowing them to move under their own control. The failure to allow persons to experience and practice appropriate movement prevents necessary changes in the nervous system to promote relearning of typical patterns. There is, therefore, a need for a rehabilitation device that provides passive assistance, supports a person according to his/her abilities and allows the person to move using his/her own muscle power.
Gravity balancing is often used in industrial machines to decrease the required actuator efforts during motion, but has rarely been used for assistive rehabilitation devices. A machine is said to be gravity balanced if joint actuator torques are not needed to keep the system in equilibrium in any configuration. Gravity balancing is a useful principle that can assist a user in walking and in STS activity. In STS motion the required joint torques are due to gravity, passive muscle forces, and inertia. Because STS movement is relatively slow, the joint torque due to gravity is the most dominant. A gravity-balancing apparatus does not require power and keeps the human body in neutral equilibrium during the entire range of motion, reducing the amount of effort needed for the motion. A gravity-balancing apparatus may be used as a functional rehabilitative aid, a training device, or an evaluation tool for the study of specific types of motion.